Observations additional to those previously reported (34) on boutons terminaux and club endings on Mauthner cell lateral dendrites, primarily as seen in sections of permanganate-fixed material, are described. Certain new findings on OSO4-fixed endings are also included. The boutons terminaux are closely packed in the synaptic bed with ∼ 100 to 150 A gaps between their contiguous unit membranes and a few interspersed glial extensions. Their synaptic membrane complexes (SMC) appear as pairs of unit membranes separated by ∼ 100 to 150A clefts. They contain many vesicles and unoriented mitochondria, but no neurofilaments. The club endings after KMnO4 fixation are, as after OSO4 fixation (34), again seen surrounded by a layer of extracellular matrix material. These endings contain relatively few synaptic vesicles, a few unit membrane limited tubules ∼ 300 A in diameter, and mitochondria oriented perpendicular to the SMC. Neurotubules and neurofilaments are not clearly seen. These components are also virtually absent in the Mauthner cytoplasm. No ribosomes are seen in the KMnO4-fixed material. The unit membranes of the SMC of club endings show up clearly in essentially the same junctional relations described after formalin-OSO4 fixation (34). In addition, the synaptic discs in transverse section show a central beading repeating at a period of ∼ A associated with scalloping of the cytoplasmic surfaces. In oblique views, dense lines are seen repeating at a period of ∼ 90 A. In frontal views a hexagonal array of close-packed polygonal facets is seen. These repeat at a period of ∼ 95 A. Each has a central dense spot <25 A in diameter. Similar subunits are seen in the unit membranes of synaptic vesicles.

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